1. Field of the Invention
The present invention relates to position tracking systems. More specifically, the present invention relates to methods and apparatus for using LORAN-C signals for locating tracked units in a localized area in two dimensions.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
In the field of position location and tracking systems, a need has long existed in the art for a system capable of accurately locating units in urban and marine areas in an economical manner. In particular, conventional LORAN based position tracking systems generally provide only moderate accuracy. Further, such position tracking systems do not accurately track units, such as emergency vehicles, in electrically noisy environments. Conventional LORAN systems are degraded in noisy environments since such conventional systems are designed primarily for use in low noise navigation systems. Therefore, the precision available from conventional position tracking systems has been limited.
Generally, conventional LORAN systems operate by measuring the difference in time of arrival of signals from two or more LORAN stations. LORAN stations are often 500-to-700 miles from a unit being tracked, such as a ship. If a ship is entering a harbor, conventional LORAN is often not sufficiently accurate, in short distance measurements, to determine the position of the ship. This is due to uncertainty in the electrical distance from the LORAN stations to the tracked unit. Thus, accuracy in LORAN-C systems continues to be a problem.
In conventional LORAN systems, the receiver located on the tracked unit, calculates the location of the tracked unit. This is accomplished by causing the receiver to process the signals propagated by several LORAN transmitters and by determining a time difference of arrival (.delta.t) between the several signals. The time differences (.delta.t) are then mathematically manipulated to provide a position which represents an intersection of two hyperbolas which indicates the location of the tracked unit.
Generally, position tracking systems, heretofore designed for the intelligence gathering art, were too expensive for commercial and military training applications. Other approaches, such as that of the Vehicle Locating System (VLS), require a significant initial investment to emplace either satellite or terrestrial repeater stations. For example, the Geostar LINK ONE system, among others, has utilized vehicular LORAN-C receivers in each tracked unit for determining the unit's position. Thereafter, the computed position is datalinked from the tracked unit back to a central station. Such a system requires a LORAN-C processor in every unit. Since signal processing is required at the receiving station located at each of the tracked units, the equipment costs at each of the units is high. Further, since the LORAN transmitting station is potentially hundreds of miles away, significant errors (as much as five-hundred meters) are often encountered. Additionally, it is well known that the signal-to-man-made noise ratio in urban areas is weak, further decreasing accuracy.
Another method of interest is one employed in the art of meteorology. In that technique, a 100 KHz signal transmitted by a LORAN-C transmitter is received by a tracked unit, in this case a weather balloon incorporating a receiver RF amplifier. The receiver RF amplifier receives the 100 KHz signal from the LORAN-C transmitter and transponds the signals to an Ultra High Frequency (UHF) carrier signal for broadcast to a central ground station. There, the frequency of the signal is reconverted back to 100 KHz wherein the reconverted signal is delivered to a LORAN-C receiver.
The function of the weather balloon incorporating the receiver RF amplifier is to collect and process meteorological data such as temperature, pressure, wind speed, humidity and the like. Further, the position of the weather balloon is of significance and thus the receiver mounted on the balloon can receive multiple LORAN signals and transpond each of the signals to the ground station. Upon receipt, a calculation involving the time difference of reception is performed on the signals which determines the location of the balloon. It is noted that such a system operates in the manner of a conventional LORAN system. There is no attempt to receive the LORAN signal directly at the meteorological ground station.
Using the above-described techniques, an accuracy of only 200-300 meters could be achieved which is far below what is desirable for training ranges and emergency vehicle locators. It has been known for a shore based receiver station to monitor the same transmitted LORAN-C signals as those monitored by ships in the harbor. Under these circumstances, any measured differences from the known position of the shore based station were broadcast to the ships and other users of the position information. When corrections were introduced, the accuracy of the position tracking system was improved to a range of from ten to fifteen meters. However, this was an ideal situation positioned over water which is characterized by a lower electrical noise environment than that exhibited by an urban environment. Thus, the results reached in this ideal situation would not likely be repeated in the electrically noisy urban environment.
Additionally, in the traditional position tracking systems employing LORAN-C signals, generated time measurements were not based relative to the position of a local base station. Thus, inaccuracies resulted from attendant propagation anomalies.
Finally, existing prior art receiver systems failed to make full use of all the energy in the received LORAN-C signal.
Thus, there is a need in the art for an improvement in position tracking systems that utilize LORAN-C signals which provides higher precision in locating units by making time measurements relative to a local base station. In addition, there is a need for a system which provides improved system sensitivity, permits the tracking system to avoid traditional inaccuracies caused by propagation anomalies, and permits operation in an electrically noisy environment so that emergency units can be successfully tracked. Ideally, the system would allow for optimum use of the transponded signal energy by employing correlation detection of the transponded signal. The system should provide a more efficient system than prior systems and operate more economically.